This is a creation story, so it opens with a man we shall call Adam alone in his bachelor paradise. Well, not quite paradise: In the beginning, there was no light. His Eden was a raw loft, jungly with pipes and wires, that had been unevenly cobbled together from two adjacent Tribeca warehouses. The neighborhood, in those days—some 30 years ago—was still a frontier whose gritty charm and cheap square footage were just beginning to attract adventurous bohemians. Adam had staked his pioneer’s claim to a “penthouse”—i.e., a pull-down ladder led to a corrugated potting shed in the shadow of a water tower. His living floor was divided into a warren of acutely angled rooms—pizza slices—with seven-foot ceilings. And then there was the windowless kitchen: a platform rising from its lagoon of darkness like an atoll of black Formica.

One day a delicate beauty whom we shall call Eve enters the story. No rib surgery was involved: A taxi left her at the unprepossessing entrance to the building. (Little did she know that she would still be living at the same address some 15 years later.) The scene upstairs, she says now, might have been an outtake from the movie Big. Her future mate, a young media executive and model-builder (he constructed his first computer from a mail-order kit), showed her his impressive train collection. A maze of tracks crisscrossed the old maple floors. He deftly skirted them on roller blades. “If this guy has bunk beds,” Eve told herself, “I’m out of here.”

“I’m allergic to the precious,” remarks the wife cheerfully. “Everything had to withstand kids, cats and spaghetti sauce.”

Domestic happiness, fortunately for New Yorkers, isn’t entirely dependent on natural light. The romance endured, and in time the couple were blessed with a lively, train-loving son. When he had reached the age of reason (he is now nine), his mother decided that a major renovation was overdue. Call it temptation if you’re a prude, but no serpent prompted her revolt. It was partly the cat hair that collected in all those pointy corners, and the ’70s kitchen, and her yearning for a better view. “I knew it had to be whole hog or nothing,” she says, “although my husband, to be honest, needed some persuading.”

The virtuosity of her design team went a long way toward reassuring a reluctant mate. The SoHo-based architect, Robert Kahn, is a Modernist with a stellar client list of artists, actors and directors, many of them the couple’s friends. Stephen Sills and James Huniford, the interior designers, have known the family socially and professionally for a decade. Among their gifts is a perfect pitch for color and proportion. “Our mandate was simplicity,” Sills explains. “Our clients weren’t looking for an identity, because they have one. They wanted a relaxed environment of unpretentious luxury where warmth was the given.”

Another given was the landmarked status of the building. Kahn spent nearly three years fine-tuning the blueprints and accommodating the stringent code requirements before construction began. The wife, he says, “is an articulate realist who never missed a site meeting. Some clients seem to forget they have children. She kept the project focused on the way her family actually lives. The challenge was to retain the scale and spirit of a loft—a certain grandeur—while incorporating the amenities of an apartment with a private master wing, an intimate family room and a boy’s paradise, with a secret chamber and lots of floor space, for the son.”

His solution was to demolish the greenhouse and raise the roof two stories to create a light-flooded central atrium with clerestory windows on three sides. A narrow mezzanine, like the bridge of a ship, opens to a grass-covered terrace with glorious views. The husband’s trains circle the atrium on tracks cantilevered from the railings. Like chapels off an apse, the major public rooms are visible from the living area,which is grounded by a massive hearth. “We lowered the ceilings in the entrance gallery,” Kahn continues, “to enhance the drama of emerging into the light.”

While construction was under way, the family moved to their boat, moored on the Hudson. Sills and Huniford had refurbished its interiors, a project that informed their approach to the loft. “Nothing on a vessel should be gratuitous,” Sills observes. “The décor promotes a sense of discipline and stability. In that sense, the efficient glamour of the residence has a yachtlike feel.” The wife asked for a sunny, monochrome palette—no pattern or prints—and for minimal furnishings she could rearrange. For the living room, Sills and Huniford designed an ottoman on concealed casters; a pale, abstract rug; and deep club chairs with a Déco profile. “The wife loved the notion of Venetian plaster,” Huniford says, “which we used throughout for texture and depth, varying the nuance of the tones subtly from room to room.” The partners also suggested an eat-in kitchen rather than an isolated formal dining room, then kept it spare, with white-glass backsplashes and honed granite counters. The stone looks delicious—a creamy white with brown speckles, like French vanilla—but before she agreed to it, the wife steeped a sample in red wine. “I’m allergic to the precious,” she says cheerfully. “Everything had to withstand kids, cats and spaghetti sauce.”

Wherever possible, the architect and designers recycled elements or objects that their clients loved—the kitchen table, once the wife’s writing desk; a fanciful mosaic above the husband’s tub; a pair of antique sconces in the powder room. At Adam’s urging, the team salvaged the century-old floors, which, he had argued, were too soulful to destroy. “This meant taking them up plank by plank and refinishing them by hand,” Kahn says, “though not so evenly that we erased their character. Only when we relaid them did we realize how right that decision was.”

The fluid expanse of honey-colored bare wood unifies the space visually, and its luster heightens the general radiance. But the act of saving the floors accomplished something more personal, perhaps—even literary. It related the loft’s new incarnation to its, and to the couple’s, history. Thus does the genesis of an intelligent design end like a creation story: feeling inevitable.